If you have some time to spare and are interested, look up Thanos. He's the bad guy, and then you'll see the scope of what Marvel is going for by introducing the Guardians of the Galaxy.

Ronan the Accuser will most likely be the bad guy with the most screen time, but it looks like it will be Thanos pulling his strings behind the scenes, from what I have read. They won't want too much Thanos in Guardians or it would spoil the goodness that will be Avengers 3.

I don't think Ronan was in the trailer, unless it was him for a split second in one of the parts near the end. Nebula, a secondary villain, is in it a couple times. This trailer was more to introduce the Guardians, as most people have never heard of them. I imagine the next couple trailers will give more of a glimpse as to the plot of the movie.